Get a Grainy Grip

Often robots require some sort of grasping apparatus in order to interact with the world. But simulating a hand is an incredibly difficult engineering task. The device must apply the correct amount of force—enough to hold on to and object, but not so much that it crushes it. An impressive array of grippers has been developed, but physicist Eric Brown and his colleagues at the University of Chicago, Cornell University and iRobot Corporation think one form might be both simpler and more versatile than many others: the sandbag.

“Most robots nowadays are metal, wheels, gears, batteries and claws—all hard components. People and animals are made of soft materials and we can do a lot of things that robots aren’t good at yet. So maybe we should start building soft robots,” says Brown. “Our expertise here is in the jamming of granular materials. Materials like sand can flow kind of like a liquid, but if you compact them tightly, they can be very hard and solid, like a vacuum-packed bag of coffee. We came up with gripping as a place where we could apply the physics of this transition from liquid to solid.”

Foregoing the idea of even trying to look like fingers, the team made a gripper that consists of a balloon filled with granules (in one prototype, they actually used ground coffee) that conform to the shape of a wide range of objects. As the group reports in the November 2 issue of the Proceedings of the National Academy of Sciences of the U.S.A., a vacuum tube is used to suck the air out of the membrane, compacting the granules and making the gripper rigid in shape around the object. The object can be lifted and moved, then the vacuum turned off in order to release it.

In tests with a gripper of about 4 centimeters in diameter, Brown and his colleagues were able to successfully pick up and move objects with shapes as varied as spheres, cubes, springs, jacks, foam ear plugs and screwdrivers. The gripper could pick up light bulbs and raw eggs without damaging them, lift a glass and pour water from it, or grasp a pen and draw. The device did have trouble picking up extremely thin disks or very soft objects such as cotton balls.

Applying a vacuum to reduce the volume of the grains by as little as 0.5 percent was enough to jam them together. “You don’t have to close your fingers with a precise amount of strength to hold a wine glass; there’s a lot of tolerance there,” Brown explains. “Because our gripper is soft, I can use one pressure to pick up a lot of different things without breaking them.”

The team has calculated that the gripper could theoretically be scaled up to a meter in width and be capable of lifting weights up to 10,000 kilograms. The idea of using soft fingers on robot grippers has been around for some time. However, until recently the mechanism of jamming grains was not sufficiently understood. “The part of our gripper that was really new was using the jamming transition as a way of pinching and holding onto things— the idea that you don’t actually need claws, or muscles to close your fingers around something, that we could use only the soft, granular material with just some confining pressure to do that work for us,” Brown says.

The team is exploring the idea of creating jamming-based pads on the fingers of existing robot grippers in order to expand their versatility. They also think their gripper would be good for search-and-rescue uses, where it could quickly sort through rubble piles, or for landmine removal. And they are working on creating more robust containment membranes for the gripper that won’t degrade from hard use. In fact, the device may so effectively emulate the functionality of the human hand that it might help those who have lost one. “Another application we’ve been talking a lot about is prosthetics, because the control is very simple; you wouldn’t need a lot of remaining muscle control to operate the hand itself,” Brown says. “It could be a simple push-button control.”